1. Field of the Invention
This invention relates to ballistic armor structures produced using silicon infiltration technology. More particularly, the invention relates to infiltration techniques to form ceramic-rich composite armor bodies, e.g., reaction-bonded silicon-carbide bodies.
2. Discussion of Related Art
In many armor applications, weight is not a critical factor, and traditional materials such as steel can offer some level of protection from airborne kinetic energy threats such as ballistic projectiles and shell fragments. Steel armors offer the advantage of low cost and the fact that they also can serve as structural members of the equipment into which they are incorporated. In recent decades, certain hard ceramic materials have been developed for certain armor applications. These ceramic-based armors, such as alumina, boron carbide and silicon carbide provide the advantage of being lighter in mass than steel for the same level of protection. Thus, in applications in which having an armor having the lowest possible mass is important, such as (human) body armor and aircraft armor, low specific gravity armor materials are called for. The lower the density, the greater the thickness of armor that can be provided for the same areal density. In general, a thick armor material is more desirable than a thinner one because a greater volume of the armor material can be engaged in attempting to defeat the incoming projectile. Moreover, the impact of the projectile on a thicker armor plate results in less tensile stress on the face of the plate opposite that of the impact than that which would develop on the back face of a thinner armor plate. Thus, where brittle materials like ceramics are concerned, it is important to try to prevent brittle fracture due to excessive tensile stresses on the back face of the armor body; otherwise, the armor is too easily defeated. Rather, by preventing such tensile fracture, the kinetic energy of the projectile perhaps can be absorbed completely within the armor body, which energy absorption manifests itself as the creation of a very large new surface area of the armor material in the form of a multitude of fractures, e.g., shattering.
U.S. Pat. No. 5,372,978 to Ezis discloses a projectile-resistant armor consisting predominantly of silicon carbide and made by a hot pressing technique. Up to about 3 percent by weight of aluminum nitride may be added as a densification aid. The finished product features a microstructure having an optimal grain size of less than about 7 microns. Fracture is intergranular, indicating energy-absorbing crack deflection. Moreover, the economics of manufacturing are enhanced because less expensive, less pure grades of silicon carbide can be used without compromising the structural integrity of the material.
U.S. Pat. No. 4,604,249 to Lihleich et al. discloses a composition particularly suited for armoring vehicles. The composition is a composite of silicon carbide and steel or steel alloy. Silicon and carbon particulates, optionally including silicon carbide particulates, are mixed with an organic binder and then molded to form a green body. The green body is then coked at a maximum temperature in the range of about 800° C. to about 1000° C. The temperature is then rapidly raised to the range of about 1400° C. to about 1600° C. under an inert atmosphere of at least one bar pressure. In this temperature range, the silicon and carbon react to form silicon carbide, thereby producing a porous body. The pores are then evacuated in a vacuum chamber, and the body is immersed in molten steel or steel alloy. The metal fills up the pores to produce a dense composite armor material.
U.S. Pat. No. 3,725,015 to Weaver discloses composite refractory articles that, among other applications, have utility as an armor material for protection against penetration by ballistic projectiles. These compositions are prepared by cold pressing a mixture of a powdery refractory material and about 10 to 35 parts by volume of a carbon containing substance, such as an organic binder material or elemental carbon to form a preform, heat-treating the preform to convert the carbonaceous material to carbon, and then contacting the heated preform with a molten metal bath, the bath containing at least two metals and maintained at a temperature between 1700° C. and 1900° C. The molten metal infiltrates the preform, the refractory material matrix sinters and at least one of the metallic constituents reacts with the carbon to produce a metal carbide. Because the thermal expansion coefficient of the metal mixture is close to or slightly greater than that of the refractory matrix, the composite shape cools to room temperature essentially free of cracks and residual stress. Weaver states that, while there are no rigid particle size parameters except those dictated by the properties desired in the final product, a maximum size of about 350 microns for the particles of the powdered materials that make up the mixture to be pressed is preferred.
U.S. Pat. No. 3,796,564 to Taylor et al. discloses a hard, dense carbide composite ceramic material particularly intended as ceramic armor. Granular boron carbide is mixed with a binder, shaped as a preform, and rigidized. Then the preform is thermally processed in an inert atmosphere with a controlled amount of molten silicon in a temperature range of about 1500° C. to about 2200° C., whereupon the molten silicon infiltrates the preform and reacts with some of the boron carbide. For armor applications, Taylor places a limit of 300 microns as the maximum size for the granular boron carbide component. The formed body comprises boron carbide, silicon carbide and silicon. Taylor states that such composite bodies may be quite suitable as armor for protection against low caliber, low velocity projectiles, even if they lack the optimum properties required for protection against high caliber, high velocity projectiles.
U.S. Pat. No. 3,857,744 to Moss discloses a method for manufacturing composite articles comprising boron carbide. Specifically, a compact comprising a uniform mixture of boron carbide particulate and a temporary binder is cold pressed. Moss states that the size of the boron carbide particulate is not critical; that any size ranging from 600 grit to 120 grit may be used. The compact is heated to a temperature in the range of about 1450° C. to about 1550° C., where it is infiltrated by molten silicon. The binder is removed in the early stages of the heating operation. The silicon impregnated boron carbide body may then be bonded to an organic resin backing material to produce an armor plate.
U.S. Pat. No. 3,859,399 to Bailey discloses infiltrating a compact comprising titanium diboride and boron carbide with molten silicon at a temperature of about 1475° C. The compact further comprises a temporary binder that, optionally, is carbonizable. Although the titanium diboride remains substantially unaffected, the molten silicon reacts with at least some of the boron carbide to produce some silicon carbide in situ. The boron carbide filler is generally limited to about 150 microns in size, but since the titanium diboride component does not appear to react with the silicon under the local process conditions, there is no critical upper limit of its particle size. When certain shaping techniques such as extrusion are employed, however, it is often desirable to limit the particle size to about 125 microns or less. The flexural strength of the resulting composite body was relatively modest at about 140 MPa. A variety of applications are disclosed, including personnel, vehicular and aircraft armor.
Each of the above-described armor inventions suffers from one shortcoming or another. Hot pressing is expensive and shape-limited. Hot pressed or sintered ceramics do not hold dimensional tolerances as well as reaction-bonded silicon carbide (“RBSC”). Iron matrix composite materials are heavy in relation to ceramic armors. An infiltration temperature of 2200° C. is too high, and will likely result in exaggerated grain growth. A preform containing 35 volume percent of reactable carbon will yield excessive amounts of in-situ SiC.
As the preceding synopsis of the patent literature indicates, reaction-bonded or reaction-formed silicon carbide has been proposed and evaluated as a candidate armor material as long ago as the 1960's.
In the Third TACOM Armor Coordinating Conference in 1987, Viechnicki et al. reported on the ballistic testing of a RBSC material versus sintered and hot pressed silicon carbide materials. Not only was the RBSC substantially inferior to the other silicon carbides, Viechnicki et al. came to the general conclusion that purer, monolithic ceramics with minimal amounts of second phases and porosity have better ballistic performance than multiphase and composite ceramics. (D. J. Viechnicki, W. Blumenthal, M. Slavin, C. Tracy, and H. Skeele, “Armor Ceramics—1987,” Proc. Third TACOM Armor Coordinating Conference, Monterey, Calif. (U.S. Tank-Automotive Command, Warren, Mich., 1987) pp. 27-53).
Accordingly, in spite of the price advantage of RBSC relative to sintered or hot pressed silicon carbide, what the market has preferred has been a sintered or hot pressed monolithic ceramic product. As of the beginning of the year 2000, there was little or no RBSC armor on the market.
The details of a ballistic impact event are complex. One widely held theory of defeating a ballistic projectile is that the armor should be capable of fracturing the projectile, and then erode it before it penetrates the armor. Thus, compressive strength and hardness of a candidate armor material should be important. The above-mentioned armor patent to Taylor discloses a modulus of rupture as high as 260 MPa, and furthermore states that for armor applications the strength should be at least 200 MPa.
There seems to be a consensus in the armor development community that hardness is indeed important in a candidate armor material, and in particular, that the hardness of the armor should be at least as great as the hardness of the projectile. As for the strength parameter, however, those testing armor materials have had a difficult time correlating mechanical strength (both tensile and compressive) with ballistic performance. In fact, except for hardness, there is no single static property that is a good predictor of good armor characteristics in ceramic materials. Instead, the guidance that has been provided from the armor developers to the materials developers based upon actual ballistic tests has been that candidate armors in general should possess a combination of high hardness, high elastic modulus, low Poisson's ratio and low porosity. (Viechnicki et al., p. 32-33)
U.S. Pat. No. 5,205,970 to Brun et al. discloses a method for making silicon carbide bodies having improved surface finish. In particular, reaction-bonded or reaction-formed silicon carbide bodies are produced by reactively infiltrating silicon into a porous carbonaceous preform to form a silicon carbide body. Excess infiltrant is provided to provide for complete infiltration and filling of porosity in the preform. After infiltration, the excess infiltrant appears as excess droplets on the surface of the reaction-formed body. The formed body is then placed in contact with a wicking means, such as a piece of carbon cloth and re-heated above the infiltrant melting temperature. Excess infiltrant is wicked from the surface of the body into the wicking means by capillary force. Moreover, the wicking means is readily removed from the formed body, as the excess infiltrant that had previously bonded the wicking means to the body has been removed. Any residual wicking means material remaining on the reaction-formed body can be removed by light grinding or diamond polishing.
Another patent to Weaver, namely U.S. Pat. No. 4,019,913, discloses a method for making a high strength RBSC body. Specifically, about 3 micron diameter silicon carbide powder was roll mixed with about 18 percent by weight of colloidal graphite and distilled water. After drying and screening, a tile was cold pressed to yield a silicon carbide filler loading of about 52-53 percent by volume. The preform tile was then infiltrated with silicon at a temperature of about 2070° C. in a nitrogenous atmosphere. The resulting composite body had a final density of about 3.02 g/cc and had a flexural strength of about 530-550 MPa.
Neither Brun nor Weaver '913 advances any suggestion that their materials would be suitable armor materials.
The instant inventors have re-visited RBSC as a candidate armor material because they believe that a RBSC material can be developed whose anti-ballistic performance is competitive with other armor ceramics, such as the hot pressed armors, but at reduced cost.